When I worked for Young America as a summer job while I was in college, one of the things I was working on was digitizing sail shapes. Digital photographs were taken of the sail from the deck and then I used a computer graphics program to convert the sail shapes into digital files. The digitized sails were compared to ideal digital sails for each wind strength. To digitize the sails I had to click on the photos to make dots along the draft stripes. The problem was that the black draft stripes were difficult to discern especially near the top of the sails. At the time, the sails were light colored, so black stripes were the only color choice. I made a suggestion that white stripes on black sails would be easier to digitize, but obviously the sail material could not be redesigned just so I could digitize the stripes more easily.
Now that the sails on these boats are almost black, it looks like the draft stripes are both white and black to provide more contrast. I also suspect that the software used to digitize the sail shapes is now automated, so there would no longer be a need for a college kid like me to look at photos of America's Cup sails all day long. Even at the time I was working on the sails, the team was working on permanently mounting cameras in the deck and on the spreaders to make the photos more consistent.